The present disclosure relates to subject matter contained in priority Japanese Patent Application No. 2001-147596, filed on May 17, 2001, the contents of which is herein expressly incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a hydrogen absorption alloy electrode to be used as a negative electrode of a nickel-metal hydride rechargeable battery.
2. Description of Related Art
Hydrogen absorption alloys, capable of absorbing and releasing hydrogen at and near normal temperatures and pressures, are used as the materials of negative electrodes in nickel-metal hydride rechargeable batteries. Nickel-metal hydride rechargeable batteries are high in energy density as compared to nickel-cadmium rechargeable batteries and lead batteries, and have received attention for their cleanness because of containing no toxic element.
Hydrogen absorption alloy electrodes are made of hydrogen absorption alloy particles, which are produced by crushing alloy ingots coarsely, followed by mechanical grinding in an attritor or the like so that the particles have a predetermined median diameter of, e.g., 50 xcexcm or so.
The hydrogen absorption alloys form oxide layers easily upon exposure to air. These oxide layers inhibit the absorption and release of hydrogen, thereby yielding the problem that high rate charge-discharge characteristics cannot be obtained in initial charge-discharge cycles. For improved alloy activity, there have been disclosed techniques of forming metal layers of nickel on the surfaces of the alloy particles as a hydrogen dissociation catalyst layer. An example thereof appears in Japanese Patent Laid-Open Publication No. Hei 4-137361. Specifically, the technique includes a method of treating hydrogen absorption alloys in a hot alkali solution.
Although given the nickel metal layers on their surfaces as mentioned above, the conventional hydrogen absorption alloys are greater in average particle size and relatively smaller in the surface layer thickness with respect to particle diameters. This means smaller Ni content on the surfaces of the alloy particles, smaller specific surface area per gram of alloy, and smaller surface area per electrode area of 1 cm2xc3x97thickness of 1 mm. Besides, the surface layers of the alloy particles contain less metal Ni, or are smaller in the content of metal Ni per electrode area of 1 cm2xc3x97thickness of 1 mm. This causes problems of lower activity, higher internal resistance, and poor initial battery characteristics in initial cycles.
Under the circumstances, the internal resistance has been lowered to operable values by repeating low-current charge and discharge a plurality of times on shipment for the sake of initial activation. As shown in FIG. 1, this repetition of charge-discharge cycles gradually lowers the internal resistance. Nevertheless, it takes long for a predetermined internal resistance to be reached, causing a problem of deteriorated productivity.
In light of the conventional problems mentioned above, an object of the present invention is to provide a hydrogen absorption alloy electrode which has high activity in initial, charge-discharge cycles.
A hydrogen absorption alloy electrode according to the present invention chiefly includes hydrogen absorption alloy particles each having a surface layer which mainly includes Ni metal. The alloy particles satisfy R2/R1xe2x89xa70.004 and 5 xcexcmxe2x89xa6R1xe2x89xa620 xcexcm, where R1 is a half of a median diameter of the particles and R2 is thickness of the surface layers. The yet preferable range of R1 is 5 xcexcmxe2x89xa6R1xe2x89xa612.5 xcexcm.
Consequently, the alloy particles are confined to a range of smaller particle sizes with an increase in the total surface area. Besides, the surface layers, which contain large amounts of Ni metal, are given a greater relative thickness. The result is that the surface content of Ni metal increases sufficiently. Thus, as shown in FIG. 1, the initial activation time necessary to lower the internal resistance to operable values is made shorter than heretofore, with an improvement in productivity.
Here, R1 of alloy particles 1, which have various shapes as shown in FIG. 2A, refers to the median of radii of alloy particles 1a. The alloy particles 1a are spheres having the same volumes or circles having the same cross-sectional areas which are assumed from the respective alloy particles 1. Surface layers 3 are ones different from bulk layers 2 in composition or texture, being formed by immersing alloy particles in a hot alkali aqueous solution so that misch metals, Co, Al, and Mn dissolve from the surfaces of the hydrogen absorption alloys. The surface layers 3 contain Ni metal, along with misch metal hydroxides and oxides.
The internal resistance is further lowered to shorten the initial activation time by rendering the alloy particles greater than or equal to 0.5 m2/g in specific surface area, the surface area greater than or equal to 0.28 m2 per electrode area of 1 cm2xc3x97thickness of 1 mm, the surface layers of the alloy particles greater than or equal to 1.5% by weight in the content of metal Ni, and the surface layers of the alloy particles greater than or equal to 8xc3x9710xe2x88x923 g in the content of metal Ni per electrode area of 1 cm2xc3x97thickness of 1 mm.
While novel features of the invention are set forth in the preceding, the invention, both as to organization and content, can be further understood and appreciated, along with other objects and features thereof, from the following detailed description and examples when taken in conjunction with the attached drawings.